A typical cellular wireless system includes a number of base stations that radiate to define wireless coverage areas, such as cells and cell sectors, in which mobile stations (such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped communication devices) can operate. In turn, each base station is typically coupled with equipment that provides connectivity with one or more transport networks, such as the public switched telephone network (PSTN) and/or the Internet for instance. With this arrangement, a mobile station operating within a coverage area of any base station can engage in air interface communication with the base station and can thereby communicate via the base station with various remote network entities or with other mobile stations served by the base station.
In practice, communications over the air interface between a base station and a mobile station are structured in accordance with a particular air interface protocol or “radio access technology,” with communications from the base stations to mobile stations defining a “forward link” (or downlink) and communications from the mobile stations to the base station defining a “reverse link” (or uplink). Numerous such protocols are well known in the art, and others will be developed in the future. Examples of existing protocols include CDMA (e.g., 1xRTT, 1xEV-DO), LTE, WiMAX, iDEN, TDMA, AMPS, GSM, GPRS, UMTS, EDGE, microwave, satellite, MMDS, Wi-Fi (e.g., IEEE 802.11), and Bluetooth. Each protocol may define its own procedures for initiation of calls, handoff between coverage areas, and functions related to air interface communication.
Further, each base station in a cellular wireless system has various air interface resources that the base station can allocate for use to serve mobile stations operating in its coverage area(s). For example, in each coverage area, the base station may have a limited amount of transmission power (e.g., a maximum power level of the base station's power amplifier), and the base station may need to allocate that power among concurrent communications with mobile stations. As another example, in each coverage area, the base station may have a limited frequency spectrum, and the base station may need to allocate portions of that spectrum among concurrent communications with mobile stations. And as still another example, in each coverage area, the base station may have a limited supply of spreading codes to use for encoding air interface communications, and the base station may need to allocate those codes among concurrent communications as well.
As a specific example, each coverage area in a spread spectrum system uses spreading codes to uniquely define communication channels on the air interface, and in order to preserve distinctions (orthogonality) between the codes, a limited number of such codes exists. Each sector or other coverage area of a CDMA spread spectrum system, for instance, has a limited set of “Walsh codes” that are used, along with other spreading codes, to define various air interface channels. For example, each coverage area may have just 64 Walsh codes, 128 Walsh codes, or just 256 Walsh codes. Typically, a small number of those Walsh codes are reserved for use to encode overhead control channels, while the remainder of the Walsh codes are assigned on an as-needed basis to encode bearer traffic channels for voice or data calls.
Walsh codes provide orthogonality between channels on the air interface because the cross-correlation between Walsh codes is zero when aligned. Consequently, in the absence of multipath interference or other factors, a mobile station can completely separate a forward-link channel that is encoded with a particular Walsh code from another forward-link channel that is encoded with another Walsh code, thus theoretically eliminating interference between the channels.
Given the growing demand for cellular wireless service and the limited number of Walsh codes available in each coverage area of a CDMA system, such a system may be arranged to implement one or more additional sets of spreading codes known as “quasi-orthogonal functions” (QOFs). Each set of QOFs is generated from the underlying set of Walsh codes by multiplying each base Walsh code by a QOF mask. In general, all of the QOFs thus generated by applying a given QOF mask are fully orthogonal to each other. However, any QOF generated by applying a given QOF mask is not fully orthogonal to any other QOF generated by applying a different QOF mask but is rather just partially orthogonal or quasi-orthogonal to any such other QOF. For this reason, air-interference channels encoded using QOFs that were generated by applying different QOF masks are more likely to interfere with each other.